Add return types for jsonSerialize

[arendjantetteroo]

Refs #91

Php 8 deprecations fixed.

I updated the composer to have php8 as minimum. Php8.0 has no maintenance anymore (see https://endoflife.date/php), so we might want to bump that up to 8.1 even for a new major release of this package.

Let me know what else is needed.